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Enviroment Variables

One way to allow the configuration of an application to vary depending on the environment in which it is running is to have the application look at environment variables.

In Unix, these can be defined at the command line, like this:

export DB_USER=testuser
export DB_PASSWORD=sfa8wefKH67f

Those values can be read from inside Java code in one of several ways.

Here are some examples:

Example 1: Using System

String DB_USER = System.getenv("DB_USER");

Example 2: Using ProcessBuilder

 static int getHerokuAssignedPort() {
        ProcessBuilder processBuilder = new ProcessBuilder();
        if (processBuilder.environment().get("PORT") != null) {
            return Integer.parseInt(processBuilder.environment().get("PORT"));
        return 4567; //return default port if heroku-port isn't set (i.e. on localhost)

Example 3: Multiple Environment Variables

 public static void main(String[] args) {
   HashMap<String,String> envVars =
	    getNeededEnvVars(new String []{ "FACEBOOK_CLIENT_ID",
       return a HashMap with values of all the environment variables
       listed; print error message for each missing one, and exit if any
       of them is not defined.
 public static HashMap<String,String> getNeededEnvVars(String [] neededEnvVars) {
	HashMap<String,String> envVars = new HashMap<String,String>();
	for (String k:neededEnvVars) {
	    String v = System.getenv(k);
	boolean error=false;
	for (String k:neededEnvVars) {
	    if (envVars.get(k)==null) {
		    error = true;
		    System.err.println("Error: Must define env variable " + k);
	if (error) { System.exit(1); }
	return envVars;